surface transparency

I used Gridded2DSet to draw a topography surface. I also want to draw some street lines on the surface. Each street line has some points with only x and y, I used valueToInterp method to interpolate these points to get z values, then draw these points as one line. My problem is, some segment of the line are below the surface. So I want my topography surface to be transparent so that every thing below the surface can be seen clearly. I tried the following: